It is axiomatic that the efficiency of a gasoline-powered engine can be improved by a more complete burning of the air-fuel mixture.
Carburetors are designed to homogenize the mixture by atomizing the fuel. Cylinder heads are sometimes shaped to further improve homogenization by causing the mixture to whorl inside the cylinder during the admission and compression cycles.
The addition of gasoline vapors to the mixture causes additional dispersing effects which bring more fuel molecules in direct contact with the oxygen of the air for a more thorough combustion.